The present invention relates to a method of operating a system using a heat transfer fluid comprising branched siloxanes.
Organosiloxanes and organopolysiloxanes (silicone oils), hereinafter referred to in short as “siloxanes”, have thermal stability, a wide liquid range and a low temperature dependence for their viscosity and therefore are widely used as heat transfer fluids. DE 2754705 A1 sets out the advantages of siloxanes over other heat transfer media. It is especially at very low temperatures (below −50° C.) or very high temperatures (200-450° C.) that they are superior to organic heat transfer fluids or are the solely usable nonionic heat transfer fluid at all. EP 1473346 B1 for example describes mixtures of linear and cyclic dimethylpolysiloxanes that are usable as a refrigerant down to −100° C. Furthermore, the brochure “SYLTHERM 800 Heat Transfer Fluid—Product Technical Data” from The Dow Chemical Company (CH 153-046-E-1097, October 1997) describes a linear, permethylated silicone oil (“Syltherm 800”) and reports the upper sustained use temperature to be 400° C. (closed loop, air excluded). It is further reported there that brief thermal aging up to 538° C. is achievable without appreciable decomposition.
The recited properties of siloxanes predestine their use as high temperature heat transfer fluids, for example in heat transfer systems, such as solar thermal power plants, especially in those with parabolic trough and Fresnel technology, where the heat transfer fluid is over many years exposed to high thermal aging up to 400° C. and substantial temperature variations. The use of silicone oils in solar thermal devices is described in DE 2754705 A1, U.S. Pat. Nos. 4,122,109 and 4,193,885.
WO 10103103 describes the use of polyorganosiloxanes as heat transfer fluid subject to the proviso that at least one organic moiety attached to the silicon has two carbon atoms. This fact is very disadvantageous for the use of siloxanes, since the heat transfer oil decomposes by β-H elimination when exposed to comparatively high thermal aging. Solely siloxanes without hydrogen atom β-positioned relative to the silicon are therefore suitable in a comparatively high temperature range.
The composition of siloxane mixtures is subject to rearrangement processes (equilibration) and so is temperature dependent and thereby also time dependent until equilibrium has been reached at the particular choice of temperature. Linear, permethylated silicone oils may need a few days to reach the equilibrium state, which is a mixture of linear and cyclic siloxanes, at 400° C. for example. The physical properties accordingly also change concurrently with the abovementioned process. The possible consequence is an appreciable change over time in important operating parameters of a device operated with siloxanes as heat transfer fluid, examples being the vapor pressure or the viscosity. This is disadvantageous because it may necessitate additional control engineering requirements or even extra capital expenditure to design and construct the device, or but limited utility or even complete inutility of the device over this period. The resultant cyclic siloxanes (inter alia D4) are undesirable because of their environmental and safety classifications.
The above-described rearrangement processes aside, the structure of linear siloxanes changes through disproportionation reactions. Disproportionation of linear chain members R2SiO2/2/D group leads to the formation of RSiO3/2/T and R3SiO1/2/M groups. The T groups formed are branching sites in the siloxane structure and these branchings occasion a change in the physical properties of silicone oils such as, for example, an increase in viscosity, leading to an increased level of pumping needed in order to derive from the fluid the required amount of heat. This makes the systems difficult and eventually impossible to operate.
U.S. Pat. Nos. 4,122,109 and 4,193,885 describe the addition of metal-containing stabilizers and optionally also of hydrogen-containing silicon compounds to noncyclic methylpolysiloxanes in order to eliminate the temperature-dependent change in chemical composition and thus keep the composition, and the physical properties, stable over time. However, it is clear from the examples that rearrangements cannot be fully eliminated. The fluid is reported in the related product brochure (“Syltherm 800”), mentioned above, to undergo very slow rearrangement, the equilibrium state eventually being reached after some months. This is associated with an appreciable increase in the vapor pressure. Yet since, for cost reasons, heat transfer fluids do service in solar thermal power plants for years, stabilizer addition is thus unsuitable for this application because it is in any case unable to prevent the rearrangements during this period and is even disadvantageous by dint of the increased costs for material. Nothing is said about the stabilizers influencing the disproportionation reactions.
DE 102012211258 discloses that certain mixtures of siloxanes (mixtures of linear and cyclic siloxanes) are capable of having substantially unchanging physical properties over time on thermal aging at a constant temperature even though their chemical composition changes over time until the equilibrium state. In effect, the composition of these siloxane mixtures need not correspond to the equilibrium composition at that temperature. However, disproportionation reactions cannot be eliminated in such siloxane mixtures. In addition, cyclic siloxanes (inter alia D4) are undesirable because of their environmental and safety classifications.
RU 2221826 describes the use of mixtures of linear siloxanes, especially decamethyltetrasiloxane, and branched methylsiloxanes, especially methyl(trimethylsiloxy)silane as heat transfer medium for application in a temperature range of −135° C. to 120° C. Such a low temperature range is unlikely to result in a noticeable rate of the equilibration and disproportionation.
The invention provides a method of operating a system at an operating temperature of 300° C. to 500° C. using a heat transfer fluid comprising branched siloxanes of general formula I(R3SiO1/2)w(SiO4/2)z  (I)                wherew represents integer values from 4 to 20,z represents integer values from 1 to 15,R represents methyl,        wherein        the sum total of the proportions of all siloxanes of general formulae I is not less than 95% by mass, based on the heat transfer fluid as a whole.        
The (R3SiO1/2)w and (SiO4/2)z units are known respectively as M groups and as Q groups.
Siloxanes subjected to thermal aging undergo disproportionation reactions and relatively slow reversible rearrangement processes to form, in a temperature and time dependent manner, an equilibrium state between various siloxanes. The resultant change in physical properties over time at constant temperature is disadvantageous for the application of siloxanes as heat transfer media. The invention rests on the discovery that both the mechanisms are distinctly more pronounced with linear, i.e., predominantly D-containing siloxanes, than with branched, i.e., Q-containing siloxanes. The siloxanes of general formula I and their mixtures have substantially unchanging physical properties over time.